1. Field of the Invention
The present invention relates to an engine cooling system of a water-cooling type for cooling an engine by circulation of cooling water through the engine and, more particularly, to an engine cooling system which controls the degree of cooling the engine according to engine operating conditions.
2. Description of Related Art
In conventional mainstream cooling systems of a water-cooling type to be mounted in engines, regardless of operating conditions of the engines, cooling water is generally controlled at a constant temperature of about 80xc2x0 C. by means of a thermostat. However, changing the cooling degree according to the engine operating conditions (a loaded condition on an engine, engine rotational speed, etc.) has been proved effective in reducing engine friction, increasing fuel efficiency, and improving knocking performance, and so on. Hence there have been proposed some cooling systems of a water-cooling type configured to control the cooling degree according to the engine operating conditions.
This type of the cooling system is basically arranged such that a flow rate regulating valve is controlled to allow the temperature of the cooling water circulating in a cooling water passage in an engine to approach a target water temperature (hereinafter, xe2x80x9ctarget temperaturexe2x80x9d) determined according to the engine operating conditions, thereby changeably controlling the temperature of the cooling water.
However, changes in the cooling water temperature in response to the control of the flow rate regulating valve include a delay in response. Due to this, the actual cooling water temperature would overshoot or undershoot the target temperature and have hunting with respect to the target temperature, causing problems in controllability of the cooling water temperature.
To resolve such the disadvantage, there has been proposed, for example, Japanese Patent Unexamined Publication No. 10-317965 under the title of xe2x80x9cEngine cooling water controllerxe2x80x9d. In this controller, a fixed temperature range centrally including a predetermined target temperature is set as a fine-corrected temperature range. While a cooling water temperature detected by a water temperature sensor is within the fine-corrected temperature range, the opening degree of the flow rate regulating valve is increased as the temperature of the cooling water rises or is decreased as the temperature of the cooling water lowers.
The opening degree of the flow rate regulating valve is controlled to become larger when a deviation is large between the cooling water temperature detected at each point in time and the target temperature or to become smaller when the deviation is small.
In the prior art cooling water controller, in the fine-corrected temperature range centrally including the target temperature, if the cooling water temperature is converging at the target temperature, the opening degree of the flow rate regulating valve is only controlled to increase/decrease according to the deviation amount with respect to the target temperature.
As a result, the cooling water temperature could approach the target temperature, but convergence at the target temperature is insufficient and therefore hunting is likely to remain near the target temperature. In this regard, control accuracy of the cooling water temperature with respect to the target temperature has to be further improved.
The present invention has been made in view of the above circumstances and has an object to overcome the above problems and to provide an engine cooling system with improved accuracy of control for a cooling water temperature with respect to a target water temperature.
Additional objects and advantages of the invention will be set forth in part in the description which follows and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the purpose of the invention, there is provided an engine cooling system which cools an engine by circulating cooling water in a circulation passage and controls a cooling degree of the engine according to an operating condition of the engine, the system including: a flow rate regulating valve for regulating a circulation flow rate of the cooling water, which is selectively opened and closed at a variable opening and closing speed; water temperature detection means for detecting a temperature of the cooling water circulating in the circulation passage; target water temperature calculation means for calculating a target water temperature according to the operating condition of the engine; control region setting means for setting a first temperature range centering on the calculated target water temperature as a non-control region and a second temperature range centering on the target water temperature and being larger than the first temperature range, excluding the non-control region, as a control region; opening and closing speed control means for controlling the opening and closing speed of the flow rate regulating valve according to a deviation of the cooling water temperature from the target water temperature to allow the cooling water temperature, when the detected cooling water temperature is in the set control region, to approach the non-control region; and opening degree holding means for holding the flow rate regulating valve at a current opening degree when the detected cooling water temperature is in the set non-control region.
According to the above structure, the opening degree of the flow rate regulating valve is controlled to regulate the circulation flow rate of the cooling water, thereby controlling the cooling water temperature, and hence the cooling degree of the engine is controlled. The target temperature calculation means calculates the target water temperature according to the engine operating conditions. The control region setting means sets the first temperature range centrally including the calculated target temperature as the non-control region and the second temperature range centrally including the target temperature, excluding the non-control region, as the control region. While the cooling water temperature is in the control region, the opening and closing control means controls the opening and closing speed of the flow rate regulating valve according to the deviation between the target temperature and the cooling water temperature to bring the cooling water temperature close to the non-control region. Accordingly, in the control region, the opening and closing speed of the valve becomes slow as the cooling water temperature approaches the target temperature. Thus the cooling water temperature, without overshooting or undershooting, can immediately approach the non-control region. Thereafter, when the cooling water falls within the non-control region, the holding means holds the flow rate regulating vale at the current opening degree. The cooling water temperature approaching the target temperature can converge at the target temperature without unnecessary fluctuations.
According to another aspect of the invention, there is provided an engine cooling system which cools an engine by circulating cooling water in a circulation passage and controls a cooling degree of the engine according to an operating condition of the engine, the system including: a flow rate regulating valve for regulating a circulation flow rate of the cooling water, which is selectively opened and closed at a variable opening and closing speed; water temperature detection means for detecting a temperature of the cooling water circulating in the circulation passage; target water temperature calculation means for calculating a target water temperature according to the operating condition of the engine; control region setting means for setting a first temperature range centering on the calculated target water temperature as a non-control region, a second temperature range centering on the target water temperature and being larger than the first temperature range, excluding the non-control region, as a first control region, and a temperature range different from the first and second temperature ranges, centering on the target water temperature and being larger than the second temperature range, excluding the non-control region and the first control region, as a second control region; high speed opening and closing control means for controlling the flow rate regulating valve to selectively open and close at a relatively high speed when the detected cooling water temperature is in the second control region to allow the cooling water temperature to approach the first control region; low speed opening and closing control means for controlling the flow rate regulating valve to selectively open and close at a relatively low speed when the detected cooling water temperature is in the first control region to allow the cooling water temperature to approach the non-control region; and opening degree holding means for holding the flow rate regulating valve at a current opening degree when the detected cooling water temperature is in the set non-control region.
Furthermore, according to another aspect of the invention, there is provided an engine cooling system which cools an engine by circulating cooling water in a circulation passage and controls a cooling degree of the engine according to an operating condition of the engine, the system including: a flow rate regulating valve for regulating a circulation flow rate of the cooling water; water temperature detection means for detecting a temperature of the cooling water circulating in the circulation passage; target water temperature calculation means for calculating a target water temperature according to the operating condition of the engine; opening and closing speed control means for controlling the flow rate regulating valve to selectively open and close according to the calculated target water temperature and the detected cooling water temperature, and controlling an opening and closing speed of the flow rate regulating valve according to the operating condition of the engine.